The invention relates to rollers used in a coating apparatus. In particular the invention relates to apparatus used for coating one or more viscous coating compositions as a composite layer onto a continuously moving receiving surface, such as in the manufacture of photographic films, photographic papers, magnetic recording tapes or such like.
In an apparatus designed for the production of coated webs of material, the web is conveyed through the machine by a series of rollers. As the web moves through the machine, the web entrains a layer of air termed a boundary layer. At each roller, as the web approaches, the boundary layer on the web face about to contact the roller is squeezed between the web and the roller. The increased pressure causes the web to lift off the roller, thereby causing a loss of traction and poor web steering. It is well known in the art that this problem is alleviated by forming a pattern in the roller surface such that the boundary layer of air can escape, thereby recovering good traction and conveyance. The pattern may take several forms: a random pattern (U.S. Pat. No. 4,426,757), a roller wound with spaced turnings of wire (U.S. Pat. No. 5,431,321; U.S. Pat. No. 4,427,166) or a groove pattern (U.S. Pat. No. 3,405,855).
Throughout the coating machine, individual rollers may be patterned differently, however a simple and well-known pattern that is often used is the groove pattern. This consists of a periodic series of grooves cut around the circumference of the roller where the period, depth and width of the grooves is determined by the requirements for speed of conveyance and by the web material that is being conveyed (U.S. Pat. No. 3,405,855). This groove pattern is easy to manufacture and is easy to clean should debris contaminate the grooves, and thus is particularly favoured.
It is well known in the art of coating that to improve the maximum obtainable coating speed before the onset of air entrainment, an electrostatic field may be applied at the coating point (for example, EP 390774; WO 89/05477; U.S. Pat. No. 5,609,923). In general, the web is supported by a roller at the coating point and this roller is referred to as the coating roller. It is also well known that the electrostatic field may be generated by either providing a charge on the web surfaces and grounding both the coating roller and the coating liquid (for example, EP 390774; U.S. Pat. Nos. 4,835,004; 5,122,386; 5,295,039; EP 0 530 752 A1), or by biasing the coating roller while maintaining the liquid at ground potential (for example, U.S. Pat. Nos. 3,335,026; 4,837,045; 4,864,460), or by a combination of both. In either case, a particular coating defect may arise whereby the roller pattern is transferred to the final coating (see U.S. Pat. No. 5,609,923 and U.S. patent application Ser. No. 09/212,462; filed Dec. 16, 1998 by Mark C. Zaretsky et al; entitled METHOD FOR USING A PATTERNED BACKING ROLLER FOR CURTAIN COATING A LIQUID COMPOSITION TO A WEB. This defect is herein described as electrostatic pattern transfer, however, for a grooved roller this defect is sometimes known as microgroove lines. It will be understood that the defect results in unusable product and therefore must be avoided. On certain web materials and under certain conditions therefore, an electrostatic field cannot be used to enhance coating speeds, and the coating machine must be run more slowly, so reducing productivity.
According to the present invention there is provided a roller for use in a coating machine, the roller comprising a metal core having an outer cover of dielectric material, the cover being provided with an engraved pattern, the core being provided with a second pattern having ridges under the engraved pattern in said cover and in register with the pattern in said cover, whereby an electrostatic field generated above a web supported on the roller may be made substantially uniform.
The roller design alleviates the problem of electrostatic pattern transfer, thereby expanding the applicability of electrostatic fields in the coating process.
The combination of the pattern cut in the dielectric cover and the pattern formed on the core is such that when a voltage is applied to the roller, or when a charge is applied to the web being coated, the field in the immediate vicinity of an earthed plane immediately above is substantially constant. Where the earthed plane is a liquid being coated onto the web, the fact that the field is substantially constant significantly reduces the electrostatic pattern transfer defect. In addition, the pattern cut in the dielectric cover acts in the usual way to provide an escape path for the boundary layer air carried along by the web.
For a better understanding of the present invention reference is made to the following drawings and detailed description.